test: always build multi-arch image for elastic-agent-cloud

[kruskall]

Motivation/summary

To override the cloud image the elastic-agent image is fetched and modified by replacing the apm-server binary with a local copy. The GOARCH is hardcoded to amd64 so it's not possible to use a local apm-server in an arm64 deployment in cloud.

Update make task to always build a multi-platform image and target linux/amd64 and linux/arm64.

Update Dockerfile to copy the correct binary based on TARGETARCH

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

• run make docker-override-committed-version
• run make apply
• observe elastic cloud deployment working for both arm64 and amd64 deployments

Related issues

Closes #17927

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.